Process for producing improved diazotype elements



June 4, 1968 c. M. AEBI ETAL PROCESS FOR PRODUCING IMPROVED DIAZOTYPE ELEMENTS Filed May 8, 1964 Sw EWEQQGZQ 5 ms; om ow on 8 2 moi): 6 2.52% 2050 zo @225 m2 024 @253 ES 6 Swim zESSmmE zo 2:56 6 SEE 3 a /'1 NOIJJTIOSQH INVENTOR. CLAUDE M. AEBI BY BERNARD J.SERAF|N TI 1 45, la- QJ'Z'AL.

A TTOR NE Y5 United States Patent ABSTRACT GE THE DKSCLGSURE A method for improving the resolution and density of .images obtained upon the development of thermally developable light-sensitive diazotype elements containing a resinous film-forming material comprising drying and curing said elements prior to exposure and development for at least 5 seconds at a temperature in the range of from 40 to 150 C.

The present invention relates to a new method for improving the qualities of diazotype elements. More specifically, the invention relates to a process for treating diazotype elements, especially thermal diazotype microfilm, to improve the resolution and optical density of images developed with such elements.

Thermal diazotype compositions and elements are well known in the art and broadly include diazotype compositions which may be developed by the application of heat. Conventional diazotype compositions are ordinarily developed by being contacted with an alkaline medium, such as ammonia gas or an alkaline solution.

Thermal diazotype compositions have many applications including the production of microfilm elements which are useful in photoaperture cards or the like.

Thermal diazotype compositions are also useful in making copying papers, transparencies and in many other fields. A particular advantage of thermal diazotype materials is that the apparatus and procedure for thermal development is ordinarily much less complex than is the gaseous or wet development of conventional diazotype materials with alkaline media.

It has been found, however, that many of the commonly available thermal diazotype compositions do not provide images having the degree of resolution or optical density required for certain uses, especially for microfilm applications. For example, many of these compositions yield low density images having a resolution of from about 60 to 80 lines per millimeter (l./mm.). Where there is sharp size reduction, as there is in microfilm applications, resolutions on this order may result in the loss of lines in a drawing or other graphic material which is copied. In some instances, fuzzy, indistinct images may be produced. Where the copying process is used in connection with contract work for the Federal Government and must meet the specifications of the Department of Defense or other agency, images having relatively low resolution may be unacceptable.

Therefore, it is an object of the present invention to provide a new method for producing diazotype elements having high resolution and good optical density. A more particular object of the invention is to produce improved thermal diazotype microfilm elements having superior resolution and optical density.

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The manner in which the foregoing objects and many other desirable objects and advantages are achieved in accordance with the present invention will be apparent from the following detailed description of the process considered in the light of the accompanying drawing.

In the drawing:

FIGURE 1 is a graph illustrating the improved resolution obtained according to the present method, and

FIGURE 2 is a graph illustrating the improved optical density obtained according to the present invention.

In general, the present invention comprises curing a light-sensitive diazotype composition for from 5 seconds up to about 20 minutes or longer at a temperature in the range of from about 40 to 150 C. The thermal curing is carried out prior to development and may be applied under vacuum or at normal atmospheric pressure with equal success.

In the following discussion, the term resolution refers to the number of lines per millimeter in the developed image as read from a standard resolution target, such as Buck'bee-Mears or National Bureau of Standards Target. Density (d. max.) referes to the diffuse transmission density of the developed image as measured on a densitometer in accordance with ASA Standard .pH Feb. 2, 1959.

Thermal, light-sensitive diazotype compositions of the type used in the present process generally comprise the following ingredients in the approximate amounts indicated:

Ingredient: Amount in grams Light-sensitive diazonium salt or combination of salts 0.2--4.0. Coupler(s) (optional where coupler generator is used) 0.25.0. Acid stabilizer(s) (optional) To about 15.0. ZnCl stabilizer (optional) To about 1.0. Base generator(s) (optional) 'Io about 5.0. Coupler generator(s) (optional) To about 6.0. Thiourea (optional) To about 4.0.

The light-sensitive compositions are ordinarily applied as a coating in a solution of a resinous, film-forming material or are imbibed into the substrate.

Compositions of the type described above are suitable for solution in ml. of a resin vehicle solution. Ordinarily, from about 2.0% to 30.0% by weight of a soluble film-forming resin is dissolved in sufiicient volatile solvent to produce 100 ml. of solution and the ingredients of the thermal diazotype composition are then dissolved in this vehicle solution. Thus, the amounts given above in grams also represent the approximate percent by weight of the ingredient in the final solution.

The solution is then coated on a suitable substrate, such as paper or transparent plastic, to produce a coating of up to a few mils in thickness. Where the product is to be used for the production of microfilm, a polyethylene terephthalate or other transparent plastic substrate is especially satisfactory.

The element is then ordinarily allowed to dry to evaporate the volatile solvent and the product is cut to the desired form and made ready for use.

The invention is also useful in improving light-sensitive diazonium elements produced by imbibition, i.e., by coating a solution of the diazotype composition in a volatile solvent onto the surface of a substrate and then evaporating the solvent, No resinous vehicle is used. The diazotype composition is thus coated onto and impregnated onto the surface of the substrate.

Imbibition coating is conventional in the diazotype field and may be accomplished by applying a solution of the thermally developable light-sensitive diazotype composition directly onto the surface of a substrate. Ordinarily, a small amount of an acid is included in the formulation to etch the surface of the substrate and improve imbibition.

A typical formulation for imbibition coating may comprise the following ingredients:

Ingredient: Amount in grams Light-sensitive diazonium salt or combination of salts 0.2-4.0. Couplen(s) (optional where coupler generator is used) 0.25.0. Acid stabi1izer(s) (optional) To about 15.0. ZnCl stabilizer (optional) To about 1.0. Base generator(s) (optional) To about 5.0. Coupler generator(s) (optional) To about 6.0. Thiourea (optional) To about 4.0. Etching a-cid (where necessary) To about 5.0.

The balance of the composition is made up of a suitable volatile solvent, such as methanol or ethanol, to make about 100 m1. of solution.

Solutions of the previously described type are coated onto suitable substrate materials, such as cellulose diacetate or cellulose tri-acetate and are then allowed to dry and are cut to size.

According to the present invention, the diazotype composition deposited on or imbibed in the substrate, is cured prior to development for at least about seconds at a temperature in the range of from 40 to 150 C. Less curing time is sufficient at more elevated temperatures within the foregoing range.

As a result of the pro-curing, films which previously had a resolution of about 60 l./mm. exhibited an improvement to better than 150 L/mm. and in a few optimum cases, up to 300 1./mm. In addition, the improvement in resolution is accompanied by an increase in the optical density of the image formed. While the most outstanding improvement is observed with films comprising a light-sensitive diazotype composition in a resinous, filmforming vehicle, improvement is also found in elements which have been imbibed with the diazotype composition.

The excellent results achieved in accordance with the present invention will be more fully apparent in the light of the following detailed examples. While the examples are not intended to be limiting, they do serve to illustrate certain preferred embodiments of the invention.

Example 1 A resinous vehicle solution is prepared by dissolving 15% by weight of polyvinyl butyral in methanol to form a ml. solution. To this solution are added 1.3 g. citraconic acid, 0.2 g. thiourea, .1 g. 2,3-dihydroxy naphtha- 1ene-6-sulfonic acid-sodium salt, 0.15 g p-diazodimethylaniline salt of zinc chloride and 0.04 g. of zinc chloride. The solution is coated onto a transparent polyethylene terephthalate substrate to form a film having a thickness of a few mils. The film is then allowed to dry in air at room temperature. The film element thus produced is exposed to an ultra violet light source for seconds and is developed thermally for 30 seconds at 150 C. by exposure to infra-red radiation.

The resolution and optical density of the resulting image are then measured. The resolution taken with a modified National Bureau of Standards Target is found to be 54 l./mm. and the optical density of the image measured on a Welch Densitometer containing a Wratten #99 Green Filter is found to be 0.82.

4 Example 2 A thermally developable diazotype film element is produced following the procedure of Example 1, but, before exposure, the dried film element is heated in an oven for one minute at C, The film is then exposed and developed as in Example 1. Upon measurement, the resulting image is found to have a resolution of 84 l./mm. and an optical density of 0.90.

Example 3 The procedure of Example 2 is followed, but the film is cur-ed at 100 C. for a period of 2 minutes rather than one minute. After exposure and development, the image is found to have a resolution of l./rnm. and an optical density of 1.00.

Example 4 The procedure of Example 2 is again followed, but the period of curing of the film at 100 C. is extended to 3 minutes. After exposure and development, the resulting image is found to have a resolution of l./mm. and an optical density of about 0.96.

The striking improvement in resolution as the result of the oven curing described in Examples 2, 3 and 4 is graphically illustrated in FIGURE 1 of the drawing. In the graph shown in FIGURE 1, the vertical axis is calibrated for resolution in lines per millimeter. The hori zontal axis is calibrated for time of the curing cycle at 100 C. in minutes. The values measured for materials prepared in accordance with Examples 1 through 4 are plotted as indicated by the circled points. The slope of the line defined by the data indicates the striking improvement obtained in resolution as a result of the curing step before exposure and development.

Example 5 A resinous vehicle solution is prepared by dissolving 15% by weight of polyvinyl butyral in methanol to produce 100 ml. of solution. In this solution, the following ingredients are then dissolved:

. citraconic acid thiourea zinc chloride 1.0 g. p-diazo climethylaniline salt of Zinc chloride 1.0 g. of 2,3-dihydroxy naphthalene-6-sulfonic acid-sodium salt Examples 6-12 Film elements prepared in the same manner as those described in Example 5 were then subject to development over increasing periods of time and the optical density of the resulting image was measured for each period of development. The results are tabulated below:

Example Development Optical No. Time, seconds Density Examples 13-20 Example Development Optical No. Time, seconds Density The results of Examples 5-12 and 13-20 are graphically compared in FIGURE 2 of the drawing.

In the graph of FIGURE 2, the vertical axis is graduated for optical density readings taken on a Welch Densitometer and the horizontal axis is graduated for development time in seconds. The numbers next to the circled points on the upper and lower curves on the graph correspond to the number of the examples from which the data is derived.

As can readily be seen, the oven curing of the film elements results in a significant improvement in optical density for the developed image as compared with the film elements which were air dried for 24 hours at room temperature, about ZZZ-24 C. As a corollary of this, images having much higher density can be developed much more rapidly with the oven-cured elements than with air-dried elements. For example, referring again to the graph, the density of images obtained with the cured film after development for 2.5 seconds at 100 C. is almost double the density for the corresponding uncured element.

The foregoing examples have demonstrated the outstanding advantages gained in resolution and density with the present invention, as applied to systems containing resinous vehicles. However, as shown in the following examples, similar, though less striking, advantages are realized by curing elements formed by imbibition techniques.

Example 21 A solution of the following composition is prepared:

Percent Citraconic acid 13 Thiourea 2 Dicyandiamide 1 2,3-dihydroxy naphthalene-6-sulfonic acid sodium salt 1 p-Diazo diethylaniline /2 zinc chloride 2 Formic acid 2 Methanol Balance The solution is coated on the surface of a sheet of cellulose tri-acetate and is allowed to dry. The resulting film element is exposed to ultra violet light for about 15 seconds and is developed for 30 seconds. The resolution is measured at 40 l./rnm.

Example 22 A film element is prepared as in Example 21, but, after drying, it is first cured at 100 C. for one minute before exposure and development. The resulting image is found to have a resolution of 65 L/mrn.

It is apparent from Examples 21 and 22 that a substantial improvement in image properties is also realized by curing elements formed by imbibition.

It is important to note that the benefits of the present invention are not obtained simply by air drying the film, whether in a vacuum or not. This is clearly brought out by the data presented above. Curing of the film for at least about 5 seconds at temperatures above 40 C., but below the pre-coupling temperature of the thermally developable, lightsensitive diazonium composition, usually about 150 C., is necessary to obtain the desired results. At more elevated temperatures, the curing cycle may be quite brief, for example, in the range of from 5 to 15 seconds. However, where lower temperatures are employed, longer curing periods are desirable and a period of 20 minutes or more may be employed.

A further comment on the upper temperature limit for the curing operation is in order. Some diazonium salts and coupler systems of the thermal type will undergo coupling at relatively low temperatures and so the curing temperature within the above range should be selected to avoid premature coupling reactions. Therefore, for each specific diazotype system, there will be an optimum curing time and temperature which can readily be determined by the procedures followed in Examples l-20.

Especially good results are obtained with a thermal developable diazotype composition comprising from about 10% to 15% by weight of citraconic acid, up to 3% thiourea, up to 1.0% zinc chloride, up to 2.0% dicyandiamide, from about 0.5% to 2.0% 2,3-dihydroxy naphthalene-6-sulfonic acid-sodium salt and from about 0.5% to 5.0% p-diazo dimethylaniline salt of zinc chloride and from about 2.0% to 30.0% of a resinous film-forming material dissolved in about ml. of a volatile solvent for the resin. The composition is coated onto a 2 mil thick substrate, such as polyethylene terephthalate, to produce a coating having a thickness of about 3.0 mils.

Film elements prepared in this manner are preferably dried and cured for 2 minutes at about 109 C. Upon exposure of the film through a negative and development for 60 seconds by heating at C. images are formed having an optical density of 20:02 and a resolution of 150 l./mrn.

While the curing of diazotype elements prepared in accordance with the present invention may result in a slight increase in background density, this increase is not on the order of the increase realized in image density. As a result, a substantial improvement in contrast is also realized.

In general, any conventional thermal diazotype composition may be used in the present invention. These compositions may contain various well known light-sensitive diazonium compounds and couplers.

Some of the light-sensitive diazonium compounds suitable for use in the present invention include, for ex ample, p-diazo dimethylaniline, p-diazo diethylaniline, p-diazcrN-ethyLN-hydroxy ethylaniline, p-diazo-N-ethylo-toluidine, p-diazo diethyl-m-toluidine, p-diazo-N-ethyl- N-benzylaniline, 4-diazo-l-morpholino benzene, 4-diazo- 1-1norpholino-2,S-diethoxy benzene, etc. The stabilizing salts of the diazonium compounds may be any of those normally used, such as zinc chloride. borolluoride, cadmium chloride, tin chloride, etc.

Some suitable couplers for use in the present compositions include, 2,3-dihydroxy naphthalene-6-sodium sulfonate, 7-hydroxy-1,Z-naphthimidazole, 2,4',4,4-tetrah droxy biphenyl, 3,3,5,5'-tetrahydroxy biphenyl, 3,5- resorcylic acid ethanolamide, 2 hydroxy-3-naphtholic acid amide, n-beta hydroxyethyl-Z-hydroxy naphthalene-3- carboxamide, N,N ethy1ene-bis-acet0 acetamide, tetrarnethyl-ene diamine-bis-aceto 'acetamide, etc.

The resinous compositions used in the present invention may comprise a wide variety of synthetic and natural film-forming resins dissolved in volatile solvents. For example, in addition to polyvinyl butyral, other suitable resinous film formers including, polyvinyl chloride, polyvinyl acetate, cellulose acetate, polyacrylates, polyvinylidene chloride and the like, may be employed.

Suitable volatile solvents include, for example, methanol, ethanol methyl ethyl ketone, ethylene glycol monomethyl ether, acetone, ethyl acetate, propanol, etc.

The compositions ordinarily include one or more stabilizing acids such as acetic acid, citric acid, tartaric acid, or the like. Some thermal systems include acids which are volatilized or converted to anhydrides when the film is subjected to heat during thermal development. Destruction of the acid alters the pH of the system and enables the color-forming reaction to proceed. Thermal systems also conventionally include base generators which are activated by heat. Examples of these are urea, thiourea, cyanoguanidine and the like.

It has been found that citraconic acid is an especially useful ingredient in diazonium compositions, where the film is to be developed by thermal means or by a combination of thermal and alkaline treatment. This acid acts as a stabilizer but, upon heating, it is readily converted to an anhydride with consequent raising of the pH of the system.

Thermal diazonium systems may also include a coupler generator, that is, a compound which decomposes when heated to form compounds capable of coupling with the light-sensitive diazonium salt. Cyanoguanidine functions as a coupler generator in this manner, as well as serving as a base generator. Many systems also include small amounts of zinc chloride, as a stabilizer, and thiourea.

It should further be noted that the present invention is applicable to systems which are developed by a combination of thermal and alkaline development. For example, the thermal diazo systems may first be developed by infrared irradiation and may be further developed by the application of an ammonia gas or an alkaline solution according to well-known practice. Regardless of the manner of development, benefits, in terms of improved resolution and greater density, are realized.

While the present invention has been described in terms of certain illustrative examples, it will be understood by those skilled in the art that various changes and modifications may be made in the procedures and compositions of the present disclosure Without departing from the spirit or scope of the invention as expressed in the following claims:

What is claimed is:

1. A method for improving the resolution and density of images obtained upon the development of thermally developable, light-sensitive diazotype elements containing a resinous film-forming material and a diazonium salt and a material which functions as an azo-coupling component in intimate relationship with said resinous material comprising the sequential steps of drying and curing said elements prior to exposure and development for at least seconds at a temperature in the range of from 40 to 150 C.

2. A method for improving the resolution and density of images obtained upon the development of light-sensitive diazonium elements which include a substrate and, on at least part of at least one surface of the substrate, a coating of a thermally developable, light-sensitive diazotype composition containing a resinous film-forming material and a diazonium salt and a material which functions as an azo-coupling component in intimate relationship With said resinous material comprising the sequential steps of drying and curing said coating prior to cxposure and development for at least 5 seconds at a tem perature in the range of from 40 C. to 150 C.

3. A method for improving the resolution and density of images obtained upon the development of light-sensitive diazotype elements which include a substrate and, on at least part of at least one surface of the substrate, a coating of a thermally developable, light-sensitive diazotype composition containing a resinous film-forming material and a diazonium salt and a material which functions as an aZo-coupling component in intimate relationship with said resinous material comprising the sequential steps of drying and curing said coating prior to eX- posure and development for at least 5 seconds at a temperature in the range of from 40 to 150 C.

4. The method of claim 3 wherein said coating is cured at a temperature in the range of from to C.

5. A method for improving the resolution and density of images obtained upon the development of diazotype elements containing a diazonium salt and a material which functions as an azo-coupling component formed by the imbibition coating of a substrate with a thermally developable, light-sensitive diazotype composition comprising the sequential steps of drying and curing said element prior to exposure and development for at least 5 seconds at a temperature in the range of from 40 to C.

6. A method for producing thermally developable, light sensitive diazotype elements containing a diazonium salt and a material which functions as an azo-coupling component which, upon development, yields images of improved resolution and density comprising:

dissolving a thermally developable, light-sensitive diazotype composition in a solution of a film-forming resin in a volatile solvent for said resin, coating said solution onto the surface of a substrate, evaporating said volatile solvent to form a solid coating on said substrate, and the sequential steps of drying and curing said solid coating prior to exposure and development for at least 5 seconds at a temperature in the range of from 40 to 150 C.

7. A method for producing thermally developable, light-sensitive diazotype elements containing a diazonium salt and a material which functions as an azocoupling component which, upon development, yields images of improved resolution and density comprising:

dissolving a thermally developable, light-sensitive diazotype composition in a volatile solvent to form a solution,

coating said solution onto a substrate,

evaporating said volatile solvent to deposit a coating on said substrate, and

the sequential steps of drying and curing said coating prior to exposure and development for at least 5 seconds at a temperature in the range of from 40 to 150 C.

8. A method for producing thermally developable, light-sensitive diazotype elements containing a diazonium salt and a material which functions as an azo-coupling component which, upon development, yields images of improved resolution and density, comprising:

forming a solution in a volatile solvent of from about 10% to 15% by weight of citraconic acid, up to 3% thiourea, up to 1.0% zinc chloride, up to 2.0% dicyandiamide, from about 0.5% to 2.0% of a lightsensitive diazonium salt, from about 0.5% to 5.0% of a coupler for said diazonium salt, and from about 2.0% to 30% of a resinous film-forming material, said resinous material being soluble in said solvent, coating said solution onto a substrate,

evaporating said volatile solvent to deposit a solid coating on said substrate, and

the sequential steps of drying and curing said solid coating, prior to exposure and development, for at least about 5 seconds at a temperature in the range of from 40 C. to 150 C.

References Cited UNITED STATES PATENTS (Other references on following page) UNITED STATES PATENTS Klimkowski et a1 96-49 Friest et a1. 96-49 Lambert 96-49 Baril et a1. 96-49 Kosar 96-49 Aebi 96-75 Aebi et a1. 96-75 XR 10 FOREIGN PATENTS 895,250 5/1962 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

5 J. TRAVIS BROWN, Examiner.

C. BOWERS, Assistant Examiner. 

